In recent years, rechargeable and dischargeable electric storage devices such as battery cells (e.g., a lithium ion battery cell and a nickel-metal hydride battery cell) and capacitors (e.g., an electric double layer capacitor) have been adopted as the power sources of vehicles (e.g., an automobile and a motorcycle) and various devices (e.g., a portable terminal and a laptop computer). For example, various types of battery cells have been provided. One of them is a battery cell that includes: an electrode assembly; a current collector that is electrically connected to the electrode assembly; a case that houses the electrode assembly and the current collector; an external terminal disposed outside the case; and a rivet that passes through the defining wall of the case, wherein the external terminal and the current collector are electrically connected to each other via the rivet.
Examples of the above type of battery cell include a battery cell in which a rivet and an external terminal are integrally formed with each other (cf., JP-A-2001-357833, for example) and a battery cell in which a rivet and an external terminal are connected with each other via a connection conductor (cf., JP-A-2010-97764, for example). Either of these battery cells has an inner insulating member arranged along the inner surface of a defining wall of the case and an outer insulating member arranged along the outer surface of the wall of the case. The rivet is inserted through the outer, insulating member, a through-hole formed in the defining wall of the case, the inner insulating member and the current collector, and then the end of the rivet is caulked.
Thus, the current collector and the electrode assembly connected to the current collector are secured to the case. The opposite ends of the rivet integrally hold the outer insulating member, the defining wall of the case, the inner insulating member and the current collector therebetween to apply a compressive force to the peripheral area of the through-hole in the defining wall. This allows the inner and outer insulating members to be brought into close contact with the defining wall, thereby providing seal between the inside and the outside of the case.
The case is made of a metal (e.g., stainless steel, an aluminum alloy and the like). Because of this, when the rivet is caulked as described above, the peripheral area of the through-hole in the defining wall of the case is compressed and tends to be elongated in a direction crossing the axis of the rivet, i.e., in a direction crossing the direction in which the compressive force acts.
This can result in a distortion and thinning of the peripheral area of the through-hole in the defining wall of the case. This in turn could decrease the degree of contact of the inner and outer insulating members with the peripheral area of the through-hole (the degree of seal of the cell case).
In particular, when the case is made of an aluminum alloy to reduce the weight of the cell, elongation of the defining wall caused by caulking of the rivet is significant because of the softness of an aluminum alloy compared to steel such as stainless steel. If this occurs, the seal of the cell case is greatly reduced.
Meanwhile, when the rivet is used for an electrical connection between the defining wall of the case and the electrode assembly, insulation is not necessary between the defining wall and the rivet, and therefore inner and outer insulating members are unnecessary. Even in this case, however, when the rivet has been caulked, the peripheral area of the through-hole in the defining wall may be compressed causing distortion and thinning, which could in turn result in decreased seal of the case.
This problem is not limited to battery cells, but also occurs with capacitors (e.g., a double layer capacitor and the like) as well.